There are few black colorants that may be used in high temperature applications, such as concrete or fiber cement. Black mixed metal oxides like manganese ferrite have been the pigment of choice in the past for such applications. Although this type of pigment enjoys high heat stability (up to 800-1000.degree. C.), it is expensive to produce and shows inferior tinting strength. Moreover, there are rising environmental concerns regarding the use of heavy metal ions in the manufacturing process for such pigments. In reality, the temperature requirement of the majority of these high temperature applications does not demand the high degree of temperature tolerance provided by such pigments.
Thus there exists a need for an alternative, simple, low cost black pigment for such applications.
Black iron oxide has always been considered as a suitable alternative to mixed metal oxides. It is relatively inexpensive to produce and exhibits much higher tinting strength when compared to mixed metal oxides. However, black iron oxide is not currently used in applications where high temperature is involved because of its instability towards heat and oxidation. When exposed to heat, iron (II) oxide, which is responsible for the black color in black iron oxide, is known to be oxidized to iron (III) oxide, which is red. This oxidation process is caused by oxygen transfer to the surface, and the process is facilitated by heat. To increase the heat stability of black iron oxide, oxygen supply to the oxide surface must to be diminished. This can be achieved by placing a protective coating onto the surface of the oxide.
There are some examples in prior art of such coatings. Unfortunately these coatings have their limitations. U.S. patent application Ser. Nos. 08/684,417 and 08/653,357 reveal the use of alkylphenols as surface coatings on iron oxide. These surface-bound organic molecules are effective in lowering the supply of oxygen onto the iron oxide surface and hence enhances the resistance of the iron oxide to heat. However, the alkylphenol molecules render the pigment so hydrophobic that they may not be suitable for use in applications that employ very polar media like alkaline cement.
U.S. Pat. No. 4,975,214 describes the use of silica in the range of 0.2-0.5 weight percent in the synthesis of magnetic iron oxide. U.S. Pat. No. 5,718,755 also discusses the use of an iron oxide coating with 0.5-2% weight percent of silica. However, it is shown that even a surface coating of 4 percent (by weight) of silica on the pigment alone, will not significantly raise the onset temperature of oxidation for the iron oxide.
European patent EP 305,819 discloses the preparation of a ferromagnetic metal powder comprising a ferromagnetic metal particle composed mainly of iron, a silicon compound layer formed on the surface of the ferromagnetic metal particle in such an amount that the amount of silicon is 0.1 percent to 1 percent by weight based on iron in the metal particle, and a layer containing a non-ferrous transitional metal element compound, which is formed on the silicon compound layer. In accordance with this patent, the final product is a metal powder, and not an iron oxide. Moreover, as shown in latter part of the patent, a dosage of 1 percent is not enough to raise the onset temperature significantly. Furthermore, the preparation procedure for this powder is lengthy, inefficient and involves heavy metal ion like cobalt.
United Kingdom Patent No. 1,494,746 discloses the use of an inner coating of silica and an outer coating of an aromatic carboxylic acid or salt thereof onto the surface of lead chrome pigment to increase its heat stability. However, such a method demands high dosage (at least 14 weight percent silica and 17-34 weight percent salt of an aromatic carboxylic acid) of the individual coatings. Moreover, the procedure involves the complicated sequence of pH adjustment and lengthy mixing time (up to 3 hours in one single step). Furthermore, the process is only applicable to a washed slurry. In other words, washed pigment must be re-dispersed before such a procedure can be applied. All of these requirements cause disruption in the manufacturing process and add tremendous cost to the final pigment.
Accordingly, a goal of the present invention is to increase the heat stability of black iron oxide; preferably, using a modifying agent that is inexpensive. Further, the modification process should be simple, fast and non-invasive towards the manufacturing process.